A device on a bus may acknowledge and claim a transaction if the transaction corresponds to an address within an assigned address range for that device. Such a device may be said to use active decoding or positive decoding when a check is made of the address with respect to the assigned address range.
In some cases, devices are not all of the same speed. Consequently, a bus protocol may provide for positive decoding of fast devices within a first predetermined time period and for positive decoding of slower devices within a second predetermined time period. One potential drawback of such a scheme is that a bus master must wait for the slower devices if no faster device acknowledges and claims the transaction within the first predetermined time period. Another potential drawback is that it may be desirable to dynamically add devices to the system, yet the added devices may vary in speed and the address ranges for these devices may not be known in advance.
A technique, which may simplify the bus protocol, is to permit one device to acknowledge and claim any transaction if the transaction corresponds to an address that is not acknowledged and claimed by another device. Such a device may be said to use subtractive decoding. For an example, a bridge may be used to connect a PCI (peripheral component interconnect) bus to an ISA (industry standard architecture) bus. On the PCI bus, a PCI device may acknowledge and claim a transaction, for instance, within one to three cycles. Then if no PCI device acknowledges the transaction, a subtractive decoding device may acknowledge and claim the transaction for a slower ISA device without knowing which ISA device may eventually acknowledge the transaction.
The potential drawback of such a technique is that it, too, adds an additional delay for the subtractive decoding beyond the predetermined time period in which all of the positive decoding devices must respond.